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Section 8.3 Proving Triangle Similarity by SSS and SAS 435
Essential QuestionEssential Question What are two ways to use corresponding sides
of two triangles to determine that the triangles are similar?
Deciding Whether Triangles Are Similar
Work with a partner. Use dynamic geometry software.
a. Construct △ABC and △DEF with the side lengths given in column 1 of the
table below.
1. 2. 3. 4. 5. 6. 7.
AB 5 5 6 15 9 24
BC 8 8 8 20 12 18
AC 10 10 10 10 8 16
DE 10 15 9 12 12 8
EF 16 24 12 16 15 6
DF 20 30 15 8 10 8
m∠A
m∠B
m∠C
m∠D
m∠E
m∠F
b. Copy the table and complete column 1.
c. Are the triangles similar? Explain your reasoning.
d. Repeat parts (a)–(c) for columns 2–6 in the table.
e. How are the corresponding side lengths related in each pair of triangles that are
similar? Is this true for each pair of triangles that are not similar?
f. Make a conjecture about the similarity of two triangles based on their
corresponding side lengths.
g. Use your conjecture to write another set of side lengths of two similar triangles.
Use the side lengths to complete column 7 of the table.
Deciding Whether Triangles Are Similar
Work with a partner. Use dynamic geometry software. Construct any △ABC.
a. Find AB, AC, and m∠A. Choose any positive rational number k and construct
△DEF so that DE = k ⋅ AB, DF = k ⋅ AC, and m∠D = m∠A.
b. Is △DEF similar to △ABC? Explain your reasoning.
c. Repeat parts (a) and (b) several times by changing △ABC and k. Describe
your results.
Communicate Your AnswerCommunicate Your Answer 3. What are two ways to use corresponding sides of two triangles to determine that
the triangles are similar?
CONSTRUCTING VIABLE ARGUMENTS
To be profi cient in math, you need to analyze situations by breaking them into cases and recognize and use counterexamples.
COMMON CORE
Learning StandardsHSG-SRT.B.4HSG-SRT.B.5HSG-GPE.B.5HSG-MG.A.1
Proving Triangle Similarityby SSS and SAS
8.3
436 Chapter 8 Similarity
8.3 Lesson What You Will LearnWhat You Will Learn Use the Side-Side-Side Similarity Theorem.
Use the Side-Angle-Side Similarity Theorem.
Prove slope criteria using similar triangles.
Using the Side-Side-Side Similarity TheoremIn addition to using congruent corresponding angles to show that two triangles are
similar, you can use proportional corresponding side lengths.
Using the SSS Similarity Theorem
Is either △DEF or △GHJ similar to △ABC?
A C
B
16
128
D F
E
12
96
J G
H
16
10 8
SOLUTION
Compare △ABC and △DEF by fi nding ratios of corresponding side lengths.
Shortest Longest Remaining sides sides sides
AB
— DE
= 8 —
6
CA —
FD =
16 —
12
BC —
EF =
12 —
9
= 4 —
3 =
4 —
3 =
4 —
3
All the ratios are equal, so △ABC ∼ △DEF.
Compare △ABC and △GHJ by fi nding ratios of corresponding side lengths.
Shortest Longest Remaining sides sides sides
AB
— GH
= 8 —
8
CA —
JG =
16 —
16
BC —
HJ =
12 —
10
= 1 = 1 = 6 —
5
The ratios are not all equal, so △ABC and △GHJ are not similar.
FINDING AN ENTRY POINT
When using the SSS Similarity Theorem, compare the shortest sides, the longest sides, and then the remaining sides.
Previoussimilar fi gurescorresponding partsslopeparallel linesperpendicular lines
Core VocabularyCore Vocabullarry
TheoremTheoremTheorem 8.4 Side-Side-Side (SSS) Similarity TheoremIf the corresponding side lengths of
two triangles are proportional, then
the triangles are similar.
If AB
— RS
= BC
— ST
= CA
— TR
, then △ABC ∼ △RST.
Proof p. 437
A
CBS T
R
Section 8.3 Proving Triangle Similarity by SSS and SAS 437
SSS Similarity Theorem
Given RS —
JK =
ST —
KL =
TR —
LJ
Prove △RST ∼ △JKL
Locate P on — RS so that PS = JK. Draw — PQ so that — PQ � — RT . Then △RST ∼ △PSQ by
the AA Similarity Theorem (Theorem 8.3), and RS
— PS
= ST
— SQ
= TR
— QP
. You can use the given
proportion and the fact that PS = JK to deduce that SQ = KL and QP = LJ. By the
SSS Congruence Theorem (Theorem 5.8), it follows that △PSQ ≅ △JKL. Finally, use
the defi nition of congruent triangles and the AA Similarity Theorem (Theorem 8.3) to
conclude that △RST ∼ △JKL.
Using the SSS Similarity Theorem
Find the value of x that makes △ABC ∼ △DEF.
FD
E
CA
B
8
x − 1
3(x + 1)
4 12 18
SOLUTION
Step 1 Find the value of x that makes corresponding side lengths proportional.
AB
— DE
= BC
— EF
Write proportion.
4 —
12 =
x − 1 —
18 Substitute.
4 ⋅ 18 = 12(x − 1) Cross Products Property
72 = 12x − 12 Simplify.
7 = x Solve for x.
Step 2 Check that the side lengths are proportional when x = 7.
BC = x − 1 = 6 DF = 3(x + 1) = 24
AB —
DE =?
BC
— EF
4 —
12 =
6 —
18 ✓
AB —
DE =?
AC
— DF
4 —
12 =
8 —
24 ✓
When x = 7, the triangles are similar by the SSS Similarity Theorem.
Monitoring ProgressMonitoring Progress Help in English and Spanish at BigIdeasMath.com
Use the diagram.
1. Which of the three triangles are similar?
Write a similarity statement.
2. The shortest side of a triangle similar
to △RST is 12 units long. Find the
other side lengths of the triangle.
JUSTIFYING STEPSThe Parallel Postulate(Postulate 3.1) allows you to draw an auxiliary line ⃖��⃗ PQ in △RST. There is only one line through point P parallel to ⃖ ��⃗ RT , so you are able to draw it.
FINDING AN ENTRY POINT
You can use either
AB — DE
= BC — EF
or AB — DE
= AC — DF
in Step 1.
R T
S
QPLJ
K
ZY
X
T
SR
L
NM
39 36
30
33
24
3026
24
20
438 Chapter 8 Similarity
Using the SAS Similarity Theorem
You are building a lean-to shelter starting from a tree branch, as shown. Can you
construct the right end so it is similar to the left end using the angle measure and
lengths shown?
10 ft53°53°
15 ft9 ft6 ft
A
G H
F
B C
SOLUTION
Both m∠A and m∠F equal 53°, so ∠A ≅ ∠F. Next, compare the ratios of the lengths
of the sides that include ∠A and ∠F.
Shorter sides Longer sides
AB —
FG =
9 —
6
AC —
FH =
15 —
10
= 3 —
2 =
3 —
2
The lengths of the sides that include ∠A and ∠F are proportional. So, by the SAS
Similarity Theorem, △ABC ∼ △FGH.
Yes, you can make the right end similar to the left end of the shelter.
Monitoring ProgressMonitoring Progress Help in English and Spanish at BigIdeasMath.com
Explain how to show that the indicated triangles are similar.
3. △SRT ∼ △PNQ 4. △XZW ∼ △YZX
TR
SP
N Q
24
2821
18
YZW
X
15
916
1220
Using the Side-Angle-Side Similarity Theorem
TheoremTheoremTheorem 8.5 Side-Angle-Side (SAS) Similarity TheoremIf an angle of one triangle is congruent to an
angle of a second triangle and the lengths
of the sides including these angles are
proportional, then the triangles are similar.
If ∠X ≅ ∠M and ZX
— PM
= XY
— MN
, then △XYZ ∼ △MNP.
Proof Ex. 33, p. 443
X
YZN
M
P
Y
c
l
Section 8.3 Proving Triangle Similarity by SSS and SAS 439
Proving Slope Criteria Using Similar TrianglesYou can use similar triangles to prove the Slopes of Parallel Lines Theorem
(Theorem 3.13). Because the theorem is biconditional, you must prove both parts.
1. If two nonvertical lines are parallel, then they have the same slope.
2. If two nonvertical lines have the same slope, then they are parallel.
The fi rst part is proved below. The second part is proved in the exercises.
Part of Slopes of Parallel Lines Theorem (Theorem 3.13)
Given ℓ � n, ℓand n are nonvertical.
Prove mℓ = mn
First, consider the case whereℓand n are horizontal. Because all horizontal lines are
parallel and have a slope of 0, the statement is true for horizontal lines.
For the case of nonhorizontal, nonvertical lines, draw two such parallel lines,ℓand n,
and label their x-intercepts A and D, respectively. Draw a vertical segment — BC parallel
to the y-axis from point B on lineℓto point C on the x-axis. Draw a vertical segment
— EF parallel to the y-axis from point E on line n to point F on the x-axis. Because
vertical and horizontal lines are perpendicular, ∠BCA and ∠EFD are right angles.
STATEMENTS REASONS
1. ℓ � n 1. Given
2. ∠BAC ≅ ∠EDF 2. Corresponding Angles Theorem (Thm. 3.1)
3. ∠BCA ≅ ∠EFD 3. Right Angles Congruence Theorem (Thm. 2.3)
4. △ABC ∼ △DEF 4. AA Similarity Theorem (Thm. 8.3)
5. BC — EF
= AC
— DF
5. Corresponding sides of similar fi gures are
proportional.
6. BC — AC
= EF
— DF
6. Rewrite proportion.
7. mℓ = BC
— AC
, mn = EF
— DF
7. Defi nition of slope
8. mn = BC
— AC
8. Substitution Property of Equality
9. mℓ = mn 9. Transitive Property of Equality
Triangle Similarity Theorems
AA Similarity Theorem
A
C
E F
D
B
If ∠A ≅ ∠D and ∠B ≅ ∠E,
then △ABC ∼ △DEF.
SSS Similarity Theorem
A
C
E F
D
B
If AB
— DE
= BC
— EF
= AC
— DF
, then
△ABC ∼ △DEF.
SAS Similarity Theorem
A
C
E F
D
B
If ∠A ≅ ∠D and AB
— DE
= AC
— DF
,
then △ABC ∼ △DEF.
Concept SummaryConcept Summary
x
ny
A
B
CD
E
F
440 Chapter 8 Similarity
To prove the Slopes of Perpendicular Lines Theorem (Theorem 3.14), you must prove
both parts.
1. If two nonvertical lines are perpendicular, then the product of their slopes is −1.
2. If the product of the slopes of two nonvertical lines is −1, then the lines are
perpendicular.
The fi rst part is proved below. The second part is proved in the exercises.
Part of Slopes of Perpendicular Lines Theorem (Theorem 3.14)
Given ℓ⊥ n,ℓand n are nonvertical.
Prove mℓmn = −1
Draw two nonvertical, perpendicular lines,ℓand n, that intersect at point A. Draw a
horizontal line j parallel to the x-axis through point A. Draw a horizontal line k parallel
to the x-axis through point C on line n. Because horizontal lines are parallel, j � k.
Draw a vertical segment — AB parallel to the y-axis from point A to point B on line k.
Draw a vertical segment — ED parallel to the y-axis from point E on lineℓto point D on
line j. Because horizontal and vertical lines are perpendicular, ∠ ABC and ∠ ADE are
right angles.
STATEMENTS REASONS
1. ℓ⊥ n 1. Given
2. m∠CAE = 90° 2. ℓ⊥ n
3. m∠CAE = m∠DAE + m∠CAD 3. Angle Addition Postulate (Post. 1.4)
4. m∠DAE + m∠CAD = 90° 4. Transitive Property of Equality
5. ∠BCA ≅ ∠CAD 5. Alternate Interior Angles Theorem
(Thm. 3.2)
6. m∠BCA = m∠CAD 6. Defi nition of congruent angles
7. m∠DAE + m∠BCA = 90° 7. Substitution Property of Equality
8. m∠DAE = 90° − m∠BCA 8. Solve statement 7 for m∠DAE.
9. m∠BCA + m∠BAC + 90° = 180° 9. Triangle Sum Theorem (Thm. 5.1)
10. m∠BAC = 90° − m∠BCA 10. Solve statement 9 for m∠BAC.
11. m∠DAE = m∠BAC 11. Transitive Property of Equality
12. ∠DAE ≅ ∠BAC 12. Defi nition of congruent angles
13. ∠ABC ≅ ∠ADE 13. Right Angles Congruence Theorem
(Thm. 2.3)
14. △ABC ∼ △ADE 14. AA Similarity Theorem (Thm. 8.3)
15. AD — AB
= DE
— BC
15. Corresponding sides of similar fi gures
are proportional.
16. AD — DE
= AB
— BC
16. Rewrite proportion.
17. mℓ = DE
— AD
, mn = − AB
— BC
17. Defi nition of slope
18. mℓmn = DE
— AD
⋅ ( − AB
— BC
)
19. mℓmn = DE
— AD
⋅ ( − AD
— DE
) 20. mℓmn = −1
18. Substitution Property of Equality
19. Substitution Property of Equality
20. Simplify.
x
n
j
k
y
A
B C
D
E
Section 8.3 Proving Triangle Similarity by SSS and SAS 441
Exercises8.3 Dynamic Solutions available at BigIdeasMath.com
In Exercises 3 and 4, determine whether △JKL or △RST is similar to △ABC. (See Example 1.)
3.
4.
In Exercises 5 and 6, fi nd the value of x that makes △DEF ∼ △XYZ. (See Example 2.)
5.
5
11 5x + 2
14102x − 1
D F X Z
YE
6.
10
8
3(x − 1)
4
7.5x − 1
D
F
X
Z Y
E
In Exercises 7 and 8, verify that △ABC ∼ △DEF. Find the scale factor of △ABC to △DEF.
7. △ABC: BC = 18, AB = 15, AC = 12
△DEF: EF = 12, DE = 10, DF = 8
8. △ABC: AB = 10, BC = 16, CA = 20
△DEF: DE = 25, EF = 40, FD = 50
In Exercises 9 and 10, determine whether the two triangles are similar. If they are similar, write a similarity statement and fi nd the scale factor of triangle B to triangle A. (See Example 3.)
9.
8
12
9
6
D FY
WXE
A
B
10.
24
10112°
112°18
8JT
S
R L
K
A
B
In Exercises 11 and 12, sketch the triangles using the given description. Then determine whether the two triangles can be similar.
11. In △RST, RS = 20, ST = 32, and m∠S = 16°. In
△FGH, GH = 30, HF = 48, and m∠H = 24°.
12. The side lengths of △ABC are 24, 8x, and 48, and the
side lengths of △DEF are 15, 25, and 6x.
Monitoring Progress and Modeling with MathematicsMonitoring Progress and Modeling with Mathematics
1. COMPLETE THE SENTENCE You plan to show that △QRS is similar to △XYZ by the SSS Similarity Theorem
(Theorem 8.4). Copy and complete the proportion that you will use: QR
— = — YZ
= QS
— .
2. WHICH ONE DOESN’T BELONG? Which triangle does not belong with the other three? Explain your reasoning.
86
12 3
6
4
12
18
9
6
8
4
Vocabulary and Core Concept CheckVocabulary and Core Concept Check
12
8
611
7 4
63.5
7
AJ
LK
R
TSCB
14
20
16
20
2517.5
12
10.516
A J
L
K R
T
SC
B
HSCC_GEOM_PE_08.03.indd 441HSCC_GEOM_PE_08.03.indd 441 5/29/14 8:20 AM5/29/14 8:20 AM
442 Chapter 8 Similarity
In Exercises 13–16, show that the triangles are similar and write a similarity statement. Explain your reasoning.
13.
24
18
16.5 5.5
15
5
KJH
F
G
14.
15.
2147°
30X Z
Y
47°50
35
G D
J
16.
24
12
16 1812
9
SR
Q U
T
V
In Exercises 17 and 18, use △XYZ.
12
10
13
ZX
Y
17. The shortest side of a triangle similar to △XYZ is
20 units long. Find the other side lengths of the
triangle.
18. The longest side of a triangle similar to △XYZ is
39 units long. Find the other side lengths of the
triangle.
19. ERROR ANALYSIS Describe and correct the error in
writing a similarity statement.
15
1886°
86°24 20
B P R
QC
A✗
△ABC ∼ △PQR by the SAS
Similarity Theorem (Theorem 8.5).
20. MATHEMATICAL CONNECTIONS Find the value of n
that makes △DEF ∼ △XYZ when DE = 4, EF = 5,
XY = 4(n + 1), YZ = 7n − 1, and ∠E ≅ ∠Y. Include
a sketch.
ATTENDING TO PRECISION In Exercises 21–26, use the diagram to copy and complete the statement.
4
4 8
4
3.52
2M
N
R
S
Q
P
L
91°
61°
44.5°
21. m∠LNS = 22. m∠NRQ =
23. m∠NQR = 24. RQ =
25. m∠NSM = 26. m∠NPR =
27. MAKING AN ARGUMENT Your friend claims that
△JKL ∼ △MNO by the SAS Similarity Theorem
(Theorem 8.5) when JK = 18, m∠K = 130°, KL = 16, MN = 9, m∠N = 65°, and NO = 8. Do you
support your friend’s claim? Explain your reasoning.
28. ANALYZING RELATIONSHIPS Certain sections of
stained glass are sold in triangular, beveled pieces.
Which of the three beveled pieces, if any, are similar?
3 in.
5 in.
7 in.
5.25 in.3 in.
3 in.
3 in.
4 in.
4 in.
29. ATTENDING TO PRECISION In the diagram,
MN — MR
= MP
— MQ
. Which of the statements must be true?
Select all that apply. Explain your reasoning.
1 4
32R
P
M
N
Q
○A ∠1 ≅ ∠2 ○B — QR � — NP
○C ∠1 ≅ ∠4 ○D △MNP ∼ △MRQ
30. WRITING Are any two right triangles similar?
Explain.
21
18
1427
E
D
B
AC
Section 8.3 Proving Triangle Similarity by SSS and SAS 443
31. MODELING WITH MATHEMATICS In the portion of
the shuffl eboard court shown, BC
— AC
= BD
— AE
.
C
B
A
D
E
a. What additional information do you need to show
that △BCD ∼ △ACE using the SSS Similarity
Theorem (Theorem 8.4)?
b. What additional information do you need to show
that △BCD ∼ △ACE using the SAS Similarity
Theorem (Theorem 8.5)?
32. PROOF Given that △BAC is a right triangle and D, E,
and F are midpoints, prove that m∠DEF = 90°.
B
CF
ED
A
33. PROVING A THEOREM Write a two-column proof of
the SAS Similarity Theorem (Theorem 8.5).
Given ∠A ≅ ∠D, AB
— DE
= AC
— DF
Prove △ABC ∼ △DEF
B
C
F
E
D
A
34. CRITICAL THINKING You are given two right triangles
with one pair of corresponding legs and the pair of
hypotenuses having the same length ratios.
a. The lengths of the given pair of corresponding legs
are 6 and 18, and the lengths of the hypotenuses
are 10 and 30. Use the Pythagorean Theorem to
fi nd the lengths of the other pair of corresponding
legs. Draw a diagram.
b. Write the ratio of the lengths of the second pair
of corresponding legs.
c. Are these triangles similar? Does this suggest a
Hypotenuse-Leg Similarity Theorem for right
triangles? Explain.
35. WRITING Can two triangles have all three ratios of
corresponding angle measures equal to a value greater
than 1? less than 1? Explain.
36. HOW DO YOU SEE IT? Which theorem could you
use to show that △OPQ ∼ △OMN in the portion of
the Ferris wheel shown when PM = QN = 5 feet
and MO = NO = 10 feet?
P
QN
M
O
37. DRAWING CONCLUSIONS Explain why it is not
necessary to have an Angle-Side-Angle Similarity
Theorem.
38. THOUGHT PROVOKING Decide whether each is a
valid method of showing that two quadrilaterals are
similar. Justify your answer.
a. SASA b. SASAS c. SSSS d. SASSS
39. MULTIPLE REPRESENTATIONS Use a diagram to show
why there is no Side-Side-Angle Similarity Theorem.
40. MODELING WITH MATHEMATICS The dimensions
of an actual swing set are shown. You want to create
a scale model of the swing set for a dollhouse using
similar triangles. Sketch a drawing of your swing
set and label each side length. Write a similarity
statement for each pair of similar triangles. State the
scale factor you used to create the scale model.
8 ft
14 ft
4 ft
6 ft6 ft
9 in. 18 in.
8 ft
C
E
A
B
F
D
444 Chapter 8 Similarity
Maintaining Mathematical ProficiencyMaintaining Mathematical ProficiencyFind the coordinates of point P along the directed line segment AB so that AP to PB is the given ratio. (Section 3.5)
43. A(−3, 6), B(2, 1); 3 to 2 44. A(−3, −5), B(9, −1); 1 to 3 45. A(1, −2), B(8, 12); 4 to 3
Reviewing what you learned in previous grades and lessons
41. PROVING A THEOREM Copy and complete the paragraph proof of the second part of
the Slopes of Parallel Lines Theorem (Theorem 3.13) from page 439.
Given mℓ = mn, ℓand n are nonvertical.
Prove ℓ � n
You are given that mℓ = mn. By the defi nition of slope, mℓ = BC
— AC
and mn = EF
— DF
. By
______________________, BC
— AC
= EF
— DF
. Rewriting this proportion yields __________.
By the Right Angles Congruence Theorem (Thm. 2.3), _________________. So,
△ABC ∼ △DEF by ______________________. Because corresponding angles of
similar triangles are congruent, ∠BAC ≅ ∠EDF. By _____________________, ℓ � n.
42. PROVING A THEOREM Copy and complete the two-column proof of the second part of
the Slopes of Perpendicular Lines Theorem (Theorem 3.14) from page 440.
Given mℓmn = −1, ℓand n are nonvertical.
Prove ℓ⊥ n
STATEMENTS REASONS
1. mℓmn = −1 1. Given
2. mℓ= DE
— AD
, mn = − AB
— BC
2. Defi nition of slope
3. DE — AD
⋅ − AB
— BC
= −1 3. _____________________________
4. DE — AD
= BC
— AB
4. Multiply each side of statement 3
by − BC
— AB
.
5. DE — BC
=
—
5. Rewrite proportion.
6. _________________________________ 6. Right Angles Congruence Theorem
(Thm. 2.3)
7. △ABC ∼ △ADE 7. _____________________________
8. ∠BAC ≅ ∠DAE 8. Corresponding angles of similar
fi gures are congruent.
9. ∠BCA ≅ ∠CAD 9. Alternate Interior Angles Theorem
(Thm. 3.2)
10. m∠BAC = m∠DAE, m∠BCA = m∠CAD 10. _____________________________
11. m∠BAC + m∠BCA + 90° = 180° 11. _____________________________
12. _________________________________ 12. Subtraction Property of Equality
13. m∠CAD + m∠DAE = 90° 13. Substitution Property of Equality
14. m∠CAE = m∠DAE + m∠CAD 14. Angle Addition Postulate (Post. 1.4)
15. m∠CAE = 90° 15. _____________________________
16. _________________________________ 16. Defi nition of perpendicular lines
x
ny
A
B
CD
E
F
x
n
j
k
y
A
B C
D
E
